1. Field of the Invention
The present invention relates to a non-aqueous electrochemical battery comprising an anode, a cathode, a porous separator between the anode and cathode, and a liquid electrolyte, and more particularly to a non-aqueous electrochemical battery contained within a flexible hermetic pouch such that the liquid electrolyte is substantially restricted to pores of one or both electrodes and the porous separator between them. The present invention also relates to a method and apparatus for filling such an electrochemical battery to insure that the electrolyte is substantially restricted to the pores of the electrodes and the separator.
2. Description of the Related Art
The increasing use of portable electronic devices has brought with it an increasing demand for batteries which provide more energy in smaller and lighter units. One approach to meeting these more demanding requirements, for military, commercial and consumer uses, has been to incorporate more active materials, such as lithium or lithiated carbon, as the negative electrode. Lithium batteries, in general, provide higher energy density, higher specific energy, and, usually, longer shelf-life than the traditional dry cell or alkaline batteries.
The selection of a more active negative electrode has a number of design, materials, and operational consequences. In particular, water is no longer an acceptable solvent for the electrolyte. In fact, water must be specifically excluded from the electrolyte and kept from entering the battery from the outside environment. This requirement that lithium batteries be hermetically sealed initially led to the design of battery containers made of stainless steel with glass-to-metal seals surrounding the electrical feed-throughs and requiring a welding step to effect the final hermetic seal. These battery containers are very effective at preventing the entry of moisture from the environment, but also have several disadvantages.
Stainless steel battery containers are heavy and expensive. To reduce component costs, battery containers are typically cylindrical in shape. However, cylindrical batteries do not pack efficiently when several must be combined into a multi-cell battery. A further design and cost disadvantage associated with steel containers is the requirement for a designed weakening in the steel container to allow for a controlled rupture of the battery in the event of either internal or external heating of the battery. The controlled rupture is intended to deactivate the battery to prevent its explosion and the formation of hazardous shrapnel from the steel container.
Steel or other metal containers are required for those non-aqueous batteries which contain pressurized electrolytes, such as the lithium/sulfur dioxide battery. However, the development of lithium-based primary (rechargeable) and secondary (non-rechargeable) batteries using solid positive electrodes and organic solvent-based electrolytes, which have relatively low vapor pressures at operating temperatures, has led to the development of battery containers made of flexible, typically heat-sealable, polymeric films. Such batteries are commonly referred to as "pouch" cells or batteries.
Pouch cells offer significant advantages over cells contained in metal cans. They are less expensive and lighter, and significantly safer, as the flexible container does not allow internal pressures to build to a hazardous level and do not produce hazardous metallic fragments. The flexible containers associated with pouch cells comply with the shape of internal cell components, and they also expand, contract, bend, and otherwise change shape in response to external pressure on the container surfaces. Pouch cells can also be fabricated in a wide variety of shapes to permit efficient packing of many cells into multi-cell batteries or to conform with the shape of the device being powered.
A pouch cell is typically produced by first assembling a sandwich comprising the negative electrode, the separator, and the positive electrode. This assembly may be in the form of alternating flat plates, spirally wound strips, or other configurations known in the art. For the pouch cell, it is common to form a flattened structure in which the electrodes and separator material are wound in the form of an elliptical spiral.
In a separate operation, a pouch is formed, typically from two rectangular sheets of a polymeric film, by sealing them together, typically by a heat-sealing process, along three edges. The polymer film may comprise more than one layer of film to provide the necessary barriers against the ingress of moisture and air from the outside environment, while providing the necessary inertness to attack by the electrolyte solvents. In order to simplify fabrication of the pouch and to facilitate the insertion of the electrode/separator sandwich, the pouch is typically sized such that some space is left between the outer surfaces of the electrode/separator sandwich and the inner surfaces of the pouch.
After the electrode/separator sandwich has been placed in the pouch, the open end of the pouch is closed by the insertion of a cap unit or the sealing of the fourth edge. The sealing of the pouch may involve heat-sealing and/or adhesives. The final sealing design and process must make provisions for the passage of electrical connectors from the inside to the outside of the pouch and must also make provisions for the subsequent introduction of the electrolyte solution and the final sealing of that means of introduction.
In the prior art, the introduction of the electrolyte has typically been accomplished by first evacuating the sealed pouch cell to remove air from the pouch and its contents. A predetermined volume of electrolyte is then injected into the cell. This is followed by sealing the filling tube or orifice, depending upon the design.
The pouch cell designed and filled according to the prior art will have something of a pillow shape, due to the bulging of the flexible pouch caused by the filling of the space, between the electrode/separator assembly and the inner surface of the pouch, with electrolyte solution. This bulging is undesirable in that it increases the volume of the cell over that of the active components alone, and thus reduces the energy density. The bulging also represents an unnecessary expenditure, due to the presence of unneeded electrolyte. It has also been found that the prior art design and method of filling pouch cells with electrolyte solution is inefficient in filling the pores of the electrodes and separator, where electrolyte is necessary for the production of electricity. Instead, electrolyte preferentially fills the void space around the active components rather than the pores of the electrodes and separator as required for maximum utilization of the active components.